1. Field if the Invention
This invention pertains to a combustion process and a device in which the fuel supply is provided by at least one burner equipped with at least one injector.
The invention will be described specifically for use in melting glass in glass-making ovens, particularly ovens used for making float-type flat glass or ovens used to make hollow glass containers, for example, ovens that operate opposite to the type of ovens that use regenerators (energy recovery devices). However, the invention is not necessarily limited to such applications.
2. Description of the Related Art
The majority of combustion processes of the aforementioned type, particularly those used in glass-making ovens, are confronted with problems of undesirable NOx emissions. Nox emissions are harmful to humans and to the environment. Indeed, NO2 is an irritating gas that causes respiratory ailments. Additionally, in contact with the atmosphere, these gases can gradually form acid rain. Finally, they cause photochemical pollution since in combination with volatile organic compounds and solar radiation, the NOx gases are the basis for the formation of so-called tropospheric ozone which, in increased concentration at low altitude, becomes harmful for human beings, especially when it is very hot.
All these factors mean that the standards with respect to NOx emissions are becoming increasingly restrictive. Currently, because of said standards, oven manufacturers such as those that manufacture glass-making ovens are constantly concerned with limiting the maximum level of NOx emissions, preferably at a rate less than 500 mg/M3.
The parameters that influence the production of NOx gases are known. One such parameter is temperature; beyond 1300xc2x0 C., the emission of NOx gases increases exponentially with excess air, since the concentration of NOx gases depends on the square root of that of oxygen or even the concentration of N2.
Many techniques have been proposed to reduce NOx emissions. One involves causing a reducing agent to convert the NOx gases to nitrogen. This reducing agent can be ammonia, but this has disadvantages including difficulties with storage and handling of such a product. It is also possible to use a natural gas as a reducing agent, but this has detrimental effects on the fuel consumption rate of the oven and increases the CO2 emissions.
Therefore it is preferable, although not mandatory, to avoid this technique by adopting the so-called primary measures. These measures are called xe2x80x9cprimaryxe2x80x9d because one does not attempt to destroy the NOx gases that are already formed, as in the previously described technique. Rather, one tries to prevent their formation, for example at the flame level. Additionally, these measures are simpler to implement and, consequently, more economical. They do not have to completely substitute for the aforementioned technique but can advantageously complement it. These primary measurements in general amount to an indispensable precondition for reducing the consumption of reagents of the secondary measures.
One can categorize, in a non-limiting way, the existing measures in several categories:
A primary category consists of reducing the production of NOx gases via the so-called xe2x80x9creburningxe2x80x9d technique by which one creates an air-deficient zone at the oven combustion chamber level. This technique has the disadvantages of increasing the temperature at the regenerator stack and of requiring a specific design of the regenerators and their stacks, especially in terms of airtightness and resistance to corrosion.
A second category consists of affecting the flame by reducing or preventing the formation of NOx gases at that level. To do this one can, for example, attempt to reduce the amount of excess combustion air. It is also possible to attempt to limit the temperature peaks by maintaining the flame length and to increase the volume of the flame front in order to reduce the average temperature within the flame. Such a solution is, for example, described in French patent application FR 96/08663 and international application PCT/FR/97 01244, which were filed on Jul. 11, 1996 and Jul. 9, 1997, respectively. The solution consists of a combustion process for melting glass in which the liquid fuel supply and the supply of the gas and air mixture are both brought about in such a way as to spread out periodically the liquid fuel/gas-air mixture contact and/or to increase the volume of this contact in order to reduce NOx emissions.
It is an object of the invention to provide a new combustion process and device in which the fuel used is liquid, allowing one to make the flame longer and/or to reduce the temperature peaks inside the flame in order to reduce the formation of NOx gases.
Another object of the invention is to propose a combustion process and that are adjusted to all of the existing glass-making oven configurations. This will allow one to obtain an optimal thermal transfer, particularly by providing a flame of adequate length and of sufficiently great volume in order to enhance maximum coverage of the bath of substances which can be vitrified when melted.
In order to accomplish these and other objects, the invention provides a combustion process, particularly one used for melting glass, in which the fuel supply is provided by at least one burner equipped with at least one injector that includes a liquid fuel delivery tube which has at least one internal wall and one injection fluid delivery tube arranged concentrically with respect to the liquid fuel delivery tube. Immediately before ejecting the liquid fuel is ejected from its delivery tube, it is formed into a hollow jet that substantially takes on the shape of said internal wall. This perfectly resolves the problem presented. By creating a very specific flow of liquid fuel immediately before it goes out of its delivery tube, there results an increased amount of mechanical injection of the liquid fuel by the injection fluid at its outlet from this tube, resulting in heterogeneity of the drops of the fuel, and thereby avoiding burning occurring at too high a speed, which is a source of the formation of NOx gases. Consequently, for a desired flame temperature one can allow less fuel to be delivered to the intake and therefore to the flame base, which will also reduce the risk of the formation of NOx gases.
The method according to the invention does not necessarily substitute for the existing techniques but can, if necessary, complement them quite advantageously.
According to an advantageous characteristic of the invention, the liquid fuel is ejected at a delivery driving pressure of at least 1.2 MPa.
Whatever the particular configuration of the oven in which the process of the invention is implemented, one should ensure atomization of the liquid fuel necessary to avoid too rapid a burning rate.
In a preferred manner, the liquid fuel should be ejected at a temperature between 100 and 150xc2x0 C., preferably between 120 and 135xc2x0 C. Such a temperature range allows one to introduce any kind of liquid fuel that is used in traditional units, particularly in glass-making ovens, at the required viscosity immediately before it is injected from its delivery tube. This viscosity can advantageously be at least equal to 5xc2x710xe2x88x926 m2/s, especially between 10xe2x88x925 and 2xc2x710xe2x88x925 m2/s.
According to another characteristic of the invention, the liquid fuel is ejected at an opening angle cone of at least 10xc2x0, especially between 10xc2x0 and 20xc2x0. Such values allow, independent of the geometry of the liquid fuel delivery tube and its dimensions, both the necessary systematic interference between the jet of injection fluid and the liquid fuel drops, and a dispersion of the size of these drops which is optimal, so that the resulting flame will be homogeneous in temperature over its entire length.
As for the injection fluid, one can eject it in a very advantageous manner at a flow rate of more than 40 Nm3/h. Obviously, the value of the injection fluid flow rate is correlated with that of the pressure of this fluid, a pressure that should be limited as much as possible. By having a maximum flow rate value, as previously mentioned, one could obtain a sufficient flame length for all oven configurations of existing glass-making ovens.
The invention also comprises a burner equipped with at least one injector, especially one that is capable of implementing the already-described process. This includes a liquid fuel delivery tube, of the fuel oil type, which has at least one internal wall and one injection fluid delivery tube arranged concentrically with respect to the liquid fuel delivery tube. The liquid fuel delivery tube should include at least one means for inserting the liquid fuel in the form of a hollow jet, which substantially takes on the shape of the internal wall immediately before ejection.
According to one embodiment, the liquid fuel delivery tube includes at least one cylindrical tube. In this case, the inserting means will advantageously include a nozzle that is attached, preferably via screwing, to the end of the cylindrical tube. A geometry of the nozzle which is particularly well suited for the burner in accordance with the invention includes a truncated conical, swirling chamber at its downstream end that is extended by a tip whose internal wall is cylindrical.
It should be noted that the terms xe2x80x9cdownstreamxe2x80x9d and xe2x80x9cupstreamxe2x80x9d must be understood by reference to the liquid fuel delivery direction. Therefore, the downstream end of the nozzle designates the end that is farthest from the supply source of the liquid fuel and, therefore, nearest to the place where the fuel is ejected from its delivery tube. In a particularly preferred manner, the angle xcex8 at the tip of the swirling chamber is at least 30xc2x0, preferably equal to 60xc2x0, which allows one to minimize the losses of the liquid fuel load during its delivery flow.
According to a preferred variant of the invention the inserting means includes at least one element which substantially closes the liquid fuel delivery tube and is perforated by channels, especially cylindrical ones, which are oblique with respect to the liquid fuel delivery direction. This element, because of its particular geometry, confers on the liquid fuel a flow pattern in conformity with that which precedes it and gives it a sufficiently great mechanical energy level so that it can be sprayed at the outlet from its delivery tube in the form of droplets whose size dispersion rate is optimal. The channels can advantageously be uniformly distributed over the circumference of the component.
This component has a shape that allows its insertion in the liquid fuel delivery tube and can, for example, be a cylinder, preferably with two sides that are approximately parallel to one another. The sides are preferably oriented in a direction perpendicular to the direction of the liquid fuel delivery direction.
More advantageously, the orientation of each of the channels is selected so that their generatrix will make an angle xcex1 of at least 10xc2x0, especially between 15 and 30xc2x0, and preferably equal to 20xc2x0, with the liquid fuel delivery direction. This particular orientation will allow one to obtain a synergy between all of the xe2x80x9cdividedxe2x80x9d jets of liquid fuel at their outlet from the corresponding channels so that when they strike the downstream part of the delivery tube, in particular the swirling chamber of the aforementioned nozzle, they will not interfere with one another and will work together for the creation, downstream, of a single hollow jet that assumes the shape of the internal wall.
According to an additional characteristic, the component can be installed upstream from the nozzle in an airtight manner in the liquid fuel delivery tube, preferably opposite the swirling chamber.
The injection fluid delivery tube preferably includes at least one cylindrical tube at the end of which there is attached, preferably by screwing, a section perforated by an opening in which at least one part of the nozzle in accordance with the invention is inserted. Preferably the opening of the section in the external wall of the part of the nozzle which is inserted therein is arranged concentrically. This preferred arrangement can also be produced by the aforementioned screwing which is capable of ensuring self-centering of the previously described components, that is, the opening of the section with respect to the part of the nozzle which is inserted in it.
This concentricity is advantageous to the extent that if it is not available there will be a risk of the formation of very large droplets of liquid fuel, of the fuel oil type, on the periphery of the hollow jet, which will cause incomplete combustion with an increase in carbon monoxide.
Also, it is preferable that the terminal section of the nozzle be perfectly aligned in the plane defined by the side of the section that does not have contact with the injection fuel and where the opening begins. Incorrect alignment implies modification of the aerodynamics of the liquid fuel and of the injection fluid at their outlet from their respective delivery tubes.
Advantageously, the injector in conformity with the invention is installed in an airtight manner in a section of refractory material via a sealing device which includes a plate provided with cooling fins. Such an airtight installation prevents any intake of parasitic air at the level of the downstream end of the injector, parasitic air being particularly harmful in that it will increase the oxygen content at the flame root, which comprises the hottest section of the flame.
According to another characteristic, the burner in conformity with the invention also includes an adjustable support on which the previously described injector is attached and a ventilation nozzle oriented toward the downstream end of the injector, more particularly toward the aforementioned plate. The support is preferably adjustable by inclination, by azimuth, and by translation, especially so that it can rest on the plate of the airtight device. The ventilation nozzle blows out air, allowing one to avoid excessive heating locally at the level of the downstream end of the injector.
The invention also comprises a burner equipped with at least one injector that includes a liquid fuel delivery tube, of the fuel oil type, which has at least one internal wall and one injection fluid delivery tube arranged concentrically with respect to the liquid fuel delivery tube, notable in that the liquid fuel delivery tube includes at least one diffuser.
The advantages introduced by the above-described burner are undeniable. In addition to the fact that it produces less NOx gases than previously in the combustion chamber, for example an oven, it requires a lower injection fluid flow rate. This facilitates greater and more flexible use of the gas-air mixture and, therefore, allows one to obtain better results from an energy use standpoint.
The invention applies to all types of oven configurations, particularly glass-making ovens such as loop ovens, transverse burner oven, and inversion ovens. It is used in particular to reduce the emission of NOx gases.
Finally, it greatly complements the technique described in French patent application FR 96/08663 and international application PCT/FR97/01244 mentioned earlier, a technique that belongs to the technology developed by Saint-Gobain Vitrage Company under the name xe2x80x9cFenix.xe2x80x9d